It is a well known concept that the presence of entrained gases within a flowing fluid impairs the ability of most known volumetric flow meters to accurately measure the liquid volume of the flowing fluid. In fact, the effect of these entrained gases typically results in an over-reporting of the liquid component flow rates by an amount equal to the volume of the entrained gases. As such, most standards for measuring the fiscal volumetric flow of liquids require that the liquid be completely devoid of entrained gases. A problem arises, however, when it becomes impractical to ensure that the liquid stream in question is indeed, completely devoid of free gases. Although typically less than 1% of the volume, these gas volume fraction levels can often be the primary source of error in fiscal measurement and currently, except for eliminating the entrained gases altogether, there are no other solutions available to account for errors in the fiscal measurement of liquids with entrained gases. Unfortunately, in many cases, this is not a practical solution.
This problem is of particular interest in the petroleum industry in which billions of dollars of crude oil is fiscally measured each day on its way from the well heads to the refineries. Due to the large amounts of light hydrocarbons present in crude, crude oils have historically demonstrated a propensity to “out gas” during transport, resulting in small, but unknown levels of entrained gases sometimes being present at fiscal measurement locations. Since most fiscal transactions of liquids are based on the amount of liquid, current fiscal measurement standards applied to liquids with entrained gases will result in an over-reporting of the liquid flow rate. It should be appreciated that this over-reporting of liquids applies to all known measurement devices and currently, it is believed that there exists no commercial methods for providing accurate fiscal volumetric flow measurements of slightly aerated mixtures. Although the preferred solution to addressing the disadvantages discussed hereinabove would be to completely de-aerate the liquids prior to measurement, this solution appears to be impractical.